Electric heating element



July 26, 1949. ASHTON 2,477,226

ELECTRIC HEATING ELEMENT Filed Nov. 17, 1945 2 Sheets-Sheet 1 mvan'rog z W- BY 3.4m) 2 l.

{,1 ATTORNEYS- July 26, 1949.

F ASHTON ELECTRIC HEATING ELEMENT 2 Sheets-Sheet 2 Filed Nov. 1'7, 1945 .INVENTOR ATTOR N EYE Patented July 26,1949

ELECTRIC HEATING ELEMENT Frank Ashton, Birmingham, England, assignor to Bulpitt & Sons, Limited, Birmingham, England,

a British company Application November 17, 1945, Serial No. 629,238 In Great Britain December 2, 1944 4 Claims. 1

This invention relates to electric heating elements and refers more particularly to that kind of electric heating element comprising a resistance wire, usually coiled, extending through a tube of metal, the space within which is tightly packed with a heat conducting electrically insulating material, such, for instance, as fused magnesia. or alundum, to provide electrical insulation between the resistance wire and the metal tube.

In the manufacture of such elements as hitherto practised, after inserting the resistance wire and packing the tube with the electrically insulating material, the ends of the tube have been closed and the electrically insulating material has been compacted by acting upon the exterior of the tube by swaging tools to effect a mechanical contraction thereof.

The object of the present invention is to provide an improved method of manufacture.

According to the present invention, in the manufacture of an electric heating element of the kind referred to, I employ a method which includes the step of compacting the electrically insulating material against the wall of the metal tube by centrifugal force produced by rotating the tube containing the material at high speed, thereby providing an internal electrical insulation to the metal tube.

Also according to th present invention, in the manufacture of an electric heating element of the kind referred to, I employ a method which includes the steps of placing a suitable quantity of the electrically insulating material in the tube, passing a rod having a diameter approximating to the diameter of the resistance element through the centre of the tube, rotating the tube con taining the electrically insulating material at high speed, inserting the coiled resistance element in the axial opening in the electrically insulating material, inserting insulating powder through the centre of the coil, and compacting this powder by vibrating the tube containing the insulating material, the resistance element and the insulating powder.

Referring to the drawings:

Figure 1 is a view partly in section showing an element constructed in accordance with the present invention.

Figure 2 is a view partly in section showing the first stage in the process of manufacture.

Figure 3 is a sectional view ShOWing the second stage in the process of manufacture.

Figure4 is a view partly in section showing the third stage in the process of manufacture.

Figure 5 is a sectional view showing the fourth stage in the process of manufacture.

Figure 6 is a sectional view showing the fifth stage in the process of manufacture.

The manufacture is carried out by compacting a metal tube III, which may be made of copper, and which is provided with an internal thread I l at each end connecting one end of this tube to a container l2 and connecting the other end of this tube with a vacuum chamber II. The container l2 contains insulating paste, such for instance as magnesia or alundum, and the vacuum device It is operated so as to draw this material into the tube and fill it.

Screw plugs H are then screwed into each end of the tube l0, each of these plugs having an axial hole and a plunger rod I! of the same diameter as the resistance coil to be used is pushed through the tube and forces the paste outwardly into contact with the walls of the tube and eliminates gaps. The leading end I 8 of the plunger is tapered so that as it advances from left to right it tends to force the insulating material against the walls of the tube Ill. The plunger rod is then withdrawn and the tube ill containing the insulating material is mounted in a pair of aligned rotating members I1 carried by brackets l8 and the tube l0 being reasonably tight in one of the rotating members I! and this member being power driven by a band l9 engaging a pulley 20 rotated by a power unit 2|. In this way the tube It containing the insulating paste is rotated at high speed so that a dense and even layer of paste is produced by the centrifugal force which adheres to the inside of the tube.

The element tube may be now heated slowly and evenly in a muille furnace to ensure the drying and hardening of the insulation paste. The liquid content of this paste before drying, is pref erably of a relatively volatile nature.

The plugs H are now removed and the resistance coil 22 is placed in the axial opening through the resistance material 23 in the tube 10. The lower end of the tube is now fitted with an insulating bush 24 and it is sealed with insulating paste as shown at 25.

The assembly may now be placed in a heated muiile furnace again in order to dry the sealed end.

A funnel 26 is now screwed into the opposite end of the tube i0 and in this funnel is placed a suflic-ient quantity of insulating powder 21, this insulating powder flowing down through the resistance coil and between its turns. The assemly comprising the tube In and its contents and the funnel 21 is placed in a suitable structure 28 and guided at 29, 30 for vertical movement and the insulating powder is firmly compacted by producing vertical vibrations of the assembly by the rotating eccentric 32 mounted on a shaft Ill.

The lower end of the tube I0 may be provided with a sleeve such as 33 which slides in the support 30.

After the interior of the resistance coil has thus been closely packed with insulating powder,

the funnel 26 is removed and this end of the tube is fitted with an insulating bush 34 sealed with insulating paste 35.

The assembly may now be heated in a mufile furnace to dry off this second sealed end.

by side relationship and fixed in a terminal cup- 36 which may be pressed on to the tube.

What I claim then is:

l. A method of making electric heating elements of the kind referred to including the steps of inserting insulating paste in a metal tube, fitting perforated plugs in the ends of the tube, passing a rod having a conical end through the perforations in the plugs and through the paste to pack the paste against the wall of the tube, compacting the paste against the wall of a metal tube by centrifugal force produced by rotating the tube containing the material at high speed, inserting a coiled wire resistance element in the openin formed by the rod in the paste, the external diameter of this element being equal to the diameter of the opening whereby the element is centralised and held against lateral movement in the insulating material in the tube, placing the tube in an erect position, closing the lower end of the tube by a plug, feeding electrical insulating powder into the resistance coil from the upper end of the tube and compacting said powder by vibrating the assembly, whereby a sheathed heating element is produced in which the insulation between the coiled element and the wall of the tube is compacted partly by the rod and partly by centrifugal force and the insulation within the coil and between its turns is compacted by vibration.

2. A method of making electric heating elements of the kind referred to including the steps of inserting insulating paste in a metal tube, fitting perforated plugs in the ends of the tube, passing a rod having a conical end through the perforations in the plugs and through the paste to pack the paste against the wall of the tube, compacting the paste against the wall of a metal tube by centrifugal force produced by rotating the tube containing the material at high speed, inserting a coiled wire resistance element in the opening formed by the rod in the paste, the ex ternal diameter of this element being equal to the diameter of the opening whereby the element is centralised and held against lateral movement in the insulating material in the tube, placing the tube in an erect position, closing the lower end of the tube by a plug, feeding electrical insulating powder into the resistance coil from the upper end of the tube and compacting said powder by vibrating the assembly in a direction parallel to its length, whereby a sheated heating element is produced in-which the insulation between the coiled element and the wall of the tube is compacted partly by the rod and partly by centrifugal force and the insulation within the coil and between its turns is compacted by vibration.

3. A method of manufacturing electric heating elements of the kind referred to including the steps of placing electrical insulating paste in a metal tube, plugging the ends of the tube by plugs having central'holes, passing a pointed rod through the said holes and the paste within the tube so as to pack the paste against thewall of the tube and to eliminate voids, removing said rod, rotating the tube containing the paste at high speed so as to compact the paste by centrifugal action against the wall of the tube, heating the tube and paste so as to dry the paste, removing one of the plugs, closing the end of the tube from which the said plug has been removed, sealing said end of the plug, heating the sealing material, inserting a coiled resistance wire of the same external diameter as that of the rod in the space left by the removal of said rod, mounting the tube in a vertical position in a vibrator with the sealed end at the bottom, removing the plug from the upper end of the tube, applying a funnel to the said upper end, feeding insulating powder through said funnel to fill the vacant space within the coil and between the turns of the coil, vibrating the tube to compact said powder, removing said funnel, applying a plug to the end of the tube from which the funnel has been re moved, sealing said plug to the tube, and again heating the tube to solidify said sealing material, whereby a sheathed heating element is .pro-

duced in which the insulation between the coiled element and the wall of the tube is compacted partly by the rod and partly by centrifugal force and the insulation within the coil and between its turns is compacted by vibration.

4. A method of manufacturing electric heating elements of the kind referred to including the steps of placing electrical insulating paste in a metal tube, by connecting one end of the tube to a vessel containing the insulating paste and connecting the other end of the tube to a vacuum device operating the vacuum device so as to draw the paste into the tube and fill it, plugging the ends of the tube by plugs having central holes, passing a pointed rod through the said holes and the paste within the tube so as to pack the paste against the wall of the tube and to eliminate voids, rotating the tube containing the paste at 7 high speed so as to compact the paste by centrifugal action against the wall of the tube, heating the tube and paste so as to dry the paste, removing one of the plugs, closing the end of the tube from which the said plug has been removed, inserting a coiled resistance wire in the space left by the removal of said rod, mounting the tube in a vertical position in a vibrator with the sealed end at the bottom, removing the plug from the upper end of the tube, applying a funnel to the said upper end, feeding insulating powder through said funnel to fill the vacant space within the coil, vibratin the tube to compact said powder, removing said funnel and applying a plug to the end of the tube from which the funnel has been removed.

FRANK ASHTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 459,800 Waite Sept. 22, 1891 994,355 Wirt June 6, 1911 1,359,400 Lightfoot Nov. 16, 1920 1,392,174 Kempton Sept. 27, 1921 1,733,866 Crossley Oct. 29, 1929 2,039,760 Wiegand et al. May 5, 1936 2,302,564 Megow et al Nov. 17, 1942 FOREIGN PATENTS Number Country Date 528,718 Great Britain Nov. 5, 1940 

